[Haskell-cafe] How to match on such type

[Baa]

Done it by implementing own `Eqn` class and all instances:

  class Eqn a where
    eqn :: a -> a -> Int -- ^ returns weight

auto-deriving `Eq` instances help to make exact comparison, `eqn` -
with weight.

===
Best regards, Paul

> On 5 December 2017 at 21:47, Baa <aquagnu at gmail.com> wrote:
> > Hello, All!
> >
> > I have type:
> >
> >   infixr 9 |||
> >   data a ||| b = A a|B b deriving (Eq, Data, Show)
> >
> > and usually I wraps into it values which are instances of my class
> > IsTag:
> >
> >   class IsTag a where
> >     anyTag :: a
> >
> > So, I need to check if some value wrapping by `a|||b` is equal to
> > `anyTag`, i.e.:
> >
> >   A (B (A x)) == (anyTag::TypeOf_x)  
> >   ==> True  
> 
> Do you:
> 
> a) know whether it should be wrapped as A or B?
> 
> b) how many layers down it is?
> 
> One solution is to use something like (untested):
> 
> -- | Is type @a@ equivalent to @anyTag :: x@ ?
> class EquivToTag a x where
>   tagEquiv :: a -> Proxy x -> Bool
> 
> instance {-# OVERLAPPABLE #-} EquivToTag x x where
>   tagEquiv _ _ = True
> 
> instance {-# OVERLAPPABLE #-} (EquivToTag a x) => EquivToTag (a |||
> b) x where tagEquiv _ _ = True
> 
> instance {-# OVERLAPPABLE #-} (EquivToTag b x) => EquivToTag (a |||
> b) x where tagEquiv _ _ = True
> 
> ... Except there's no way of having False here, and the two |||
> instances can't really both be there (which to pick?).
> 
> I'm not sure what the intent of this is, but would it make more sense
> to use a type family which resolves to a Constraint?
> 
> >
> > This function must be generic, ie, it can not know anything about
> > concreate TypeOf_x, only: `a|||b` and `IsTag`. How to do it???
> >
> > I added `Data` to `a|||b` and even to values which I "wrap" with
> > `a|||b` (I assumed to use `gmap*` and Co), but this does not help
> > me. Is it even possible??
> >
> > ==
> > Best regadrs, Paul
> >
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>